Scientists at Lawrence Livermore National Laboratory and the University of California have found a steady increase in sperm DNA fragmentation with increasing age, along with increases in a gene mutation that causes dwarfism. This means an older man has more chance of being infertile, fathering unsuccessful pregnancies and passing along dwarfism and possibly other genetic diseases to his children.

Earlier research had indicated that male reproductive ability gradually worsens with age, as sperm counts decline and the sperm lose motility, but in this study the researchers focused on DNA damage, chromosomal abnormalities and gene mutations in sperm. They found that sperm motility showed a high correlation with DNA fragmentation, which is associated with increased risk of infertility and a reduced probability of fathering a successful pregnancy.

Writing in the Proceedings of the National Academy of Sciences, the researchers said that the study shows that men who wait until they're older to have children are not only risking difficulties conceiving, they could also be increasing the risk of having children with genetic problems. "We know that women have a biological time clock," said researcher Brenda Eskenazi, "with an increase in risk of miscarriage and producing children with trisomy (an extra chromosome, such as in Downs syndrome) as women age, and with a seemingly abrupt end of fertility around perimenopause.

Our research suggests that men, too, have a biological time clock. Men seem to have a gradual rather than an abrupt change in fertility and in the potential ability to produce healthy offspring."

Unlike in women, the researchers found no correlation between male aging and chromosome changes that cause Down's syndrome and other manifestations of trisomies; but they did conclude, however, that some older men could be at risk for fathering children with dwarfism, and that "a small fraction of men are at increased risks for transmitting multiple genetic and chromosomal defects." Interestingly, the researchers raised the possibility that socioeconomic, dietary factors or ethnic background may also be involved in how age affects the quality of human sperm.

"Since some forms of genomic damage change with age and others don't," said researcher Andrew Wyrobek, "overall genomic sperm quality cannot be measured by any single sperm test." Dwarfism, a genetic disorder that affects bone growth, is the most common growth-related birth defect, occurring in about one in every 25,000 births. It occurs in all races and in both males and females and causes affected individuals to have very short arms and legs, limiting their full adult height to about four feet. Understanding the effects of paternal age has become more important as more men are having children at older ages. Since 1980 there has been about a 40 percent increase in 35- to 49-year-old men fathering children, and a 20 percent decrease in fathers under 30.